A standard or mil-spec lower receiver for an AR15- or M16-platform rifle uses a lower trigger guard that is a separate piece and is attachable at forward and rearward ends to standardized mounting points on the lower receiver body. The standard trigger guard mounts at its rearward end by use of a roll pin hammered or pressed through a first tab extending from the lower receiver, through a transverse opening in the trigger guard, and through a second opening in a second mounting tab on the lower receiver. The forward end has a spring detent on the right side which is inserted to releasably engage an opening in the right side forward mounting tab extending from the lower receiver.
The nature of a common roll pin (also called spring pin or tension pin) is that it has a body diameter which is larger than the hole diameter for which it is intended to be used, and a chamfer on one or both ends to facilitate starting the pin into the hole. The body of the pin is split longitudinally or helically, providing a spring action that allows the diameter of the pin to compress as it assumes the diameter of the hole. The radially outward force exerted by the pin against the hole wall retains it in the hole, making it a self-retaining fastener. Installation requires, however, that the roll pin be hammered or forcibly pressed into position. In the case of the AR15/M16-platform lower receiver, the mounting holes for the trigger guard are in relatively thin tabs extending from the lower receiver body. If care is not taken as the trigger guard roll pin is hammered or pressed into place, a mounting tab can be broken. This is one of the most common breakage points on the lower receiver, often occurring during initial assembly.
An alternative attachment means for a trigger guard that does not add unnecessary bulk, parts, or complexity and that can be installed without having to hammer or force a roll pin through the mounting tabs of a lower receiver is desired.